1. Field of the Invention
The present invention relates to input signal handling apparatus in particular to such apparatus for handling input signals to a computer and more particularly, but not exclusively, to apparatus for handling "interrupt" input signals.
2. Description of Related Art
It is common practice to provide arppartus connected to a computer with the ability to signal to the computer a change of its status which requires the running of a special program. Such signals presented to the computer are usually referred to as "interrupts". Such interrupts are graded according to their urgency and in particular may be categorised as "immediate"--that is requiring urgent attention--and "non-immediate"--that is requiring handling by a special program but not likely to result in a serious malfunction if the handling of it is delayed.
On receipt of an interrupt the computer hardware forces the computer software program currently running to be "nested" (stored in recoverable state) and causes an interrupt handling program to run to deal with the interrupt.
Such action may occur unnecessarily on numerous occasions if the number of non-immediate interrupts to be handled is high in comparison to the number of immediate interrupts. Such unnecessary actions impede the efficient running of the computer and, particularly when the computer is required to operate in real-time, reduce the loading which may be placed thereon.
In United Kingdom Pat. No. 1425173, The General Electric Company Limited (now The General Electric Company, p.l.c.) disclosed a method of handling immediate and non-immediate interrupts which reduced the number of occasions on which the computer was forced into running interrupt handling programmes and disclosed an interrupt signal handling apparatus for performing the method.